This invention relates to a method and apparatus for producing high optical quality glass articles, and more particularly, it relates to an improved mandrel for use in such glass deposition processes as the flame hydrolysis process and to a method of using this mandrel.
The production of high quality, high purity vitreous materials has always been important in optics since optical systems have required low loss, distortion free components. It has recently been found that fused silica and similar high purity glasses formed by the flame hydrolysis process can be used to advantage in the production of low attenuation optical waveguides which heretofore have not been satisfactorily formed by other glass forming techniques. In addition, because of this, high quality, flame hydrolysis-produced glasses, such as fused silica, are finding greater application in the formation of lenses, prisms and the like. Although the present invention is useful for forming various optical devices by the process of building up a layer of glass on a rotating mandrel, it will be specifically described in connection with the formation of optical waveguides by the flame hydrolysis process.
The flame hydrolysis technique has recently been employed to prepare single mode waveguides and multimode waveguides of both the step-index and graded-index type. A method of forming step-index waveguides is taught in U.S. Pat. No. 3,737,292 issued June 5, 1973, to D. B. Keck et al., and a method of forming graded-index waveguides is disclosed in U.S. Pat. application Ser. No. 239,496 filed by P. C. Schultz on Mar. 30, 1972, entitled "Method of Forming a Light Focusing Fiber Waveguide" now U.S. Pat. No. 3,826,560. The flame hydrolysis process basically consists of introducing the vapor of a hydrolyzable compound into a flame to form a glass soot which is directed toward a rotating mandrel to form a coating thereon which is referred to as a soot preform. The soot preform must be subjected to a temperature within the consolidation temperature range for a time sufficient to permit the soot particles to fuse and consolidate, thereby forming a dense glass which is free from particle boundaries. The mandrel is usually employed to support the soot preform in the consolidation furnace, but it must be removed prior to drawing the consolidated soot preform into a fiber. However, the consolidation process causes the consolidated glass to bond tightly to the mandrel. Even a slight expansion mismatch between the mandrel material and that of the consolidated glass can cause large stresses to exist in the consolidated glass after it has cooled, and such stresses can cause the consolidated dense glass to crack during removal of the mandrel. Stress can be relieved by annealing the consolidated preform-mandrel composite prior to removing the mandrel, but this increases the processing time and thus increases the cost of manufacturing optical waveguides.
An apparent solution to the expansion mismatch problem is to use a mandrel of the same material as that which is being deposited by flame hydrolysis. However, optical waveguides include core and cladding materials which have different expansion characteristics, and there is necessarily an expansion mismatch between the consolidated optical waveguide preform and the mandrel. Moreover, many optical waveguide materials, such as fused silica and doped fused silica cannot be melted and formed into suitable mandrels by conventional glass forming techniques. Therefore, mandrels for use in the formation of articles of such glasses have been made from graphite, glass ceramic, quartz, and crystalline material such as alumina, mullite and the like, which have expansion coefficients different from both the core and cladding materials.
The process of removing the mandrel from the consolidated optical waveguide preform is difficult and time consuming. Although the mandrel can be removed by grinding or drilling, it is usually removed by pumping an etchant through a hollow mandrel until the mandrel is completely dissolved. The process of manufacturing optical waveguides could be made faster and simpler by improving the mandrel removing process.